Anisotropic thermal expansion in pentacene and perfluoropentacene: Effects of molecular packing motif and fixation at the interface

作者：von Helden Leonard^*; Breuer Tobias; Witte Gregor

来源：Applied Physics Letters, 2017, 110(14): 141904.

DOI：10.1063/1.4979650

Thermal expansion coefficients of molecular solids are typically significantly larger than those of inorganic materials. Since they are furthermore highly anisotropic, the molecular arrangement and consequently the intermolecular orbital overlap strongly depend on temperature, hence also affecting the energetics of optoelectronic excitations and the efficiency of charge transfer processes. Here, we report on the precise determination of the anisotropic thermal expansion coefficients of the organic semiconductor pentacene in its solid state. We compare the thermal expansion coefficients of three different pentacene polymorphs and observe distinct differences between both pentacene bulk polymorphs and the interface-stabilized thin film phase. By comparing epitaxial films with films prepared on weakly interacting, amorphous substrates, we identify a notable influence of the substrate fixation on the thermal expansion in thin pentacene films. Furthermore, the results for pentacene are compared to the thermal expansion of perfluoropentacene, where an exceptionally large vertical thermal expansion coefficient is found in the substrate-mediated p-stacked polymorph. The present study underlines the importance of thermal expansion for the interpretation of temperature-dependent spectroscopic measurements and device characterizations since the notable changes in the unit cell geometries severely affect the intermolecular coupling and thus the excitonic energetics. Published by AIP Publishing.

• 出版日期2017-4-3